The general hypothesis of this proposed research plan is that the low-pressure baroreceptors of the cardiovascular system located in the right heart, liver, kidneys, spleen, and mesenteric beds contribute to the overall neural control of the circulation, and that the central integration of the low-pressure baroreceptor reflexes occurs in the same brain nuclei as those whch integrate the high-pressure baroreceptor reflexes, i.e., nucleus tractus solitarius and nucleus ambiguus etc. The specific aims of the proposed research are to determine: 1) the physiological characteristics of the low-pressure baroreceptors using electrophysiological afferent recording techniques; 2) the reflex effects of these receptors using efferent nerve recordings and measuring changes in cardiovascular function; 3) if a heirarchy of low-pressure reflexex exists by eliciting two or more reflexes simultaneously; 4) the central functional organization of these reflexes using metabolic brain mapping, and central recording techniques; 5) what neurotransmitter substances mediate the central integration of these reflexes, using new autoradiographic neurotransmitter mapping techniques for specific transmitter substances and the iontophoresis of transmitters into areas of the brain that are functionally activated during reflex activation. These studies are designed to fill the gap of information concerning the characterization and determination of the reflex roles of the low-pressure baroreceptors in the overall neural control of the circulation, which has not been thoroughly studied. The functional brain mapping studies coupled with the studies designed to identify the specific neurotransmitters used by the cardiovascular reflexes should result in new information concerning the central organization of the cardiovascular reflexes. Since many of the low-pressure and high-pressure baroreceptor functions are altered during clinically relevant conditions such as congestive heart failure, cardiac tamponade, orthostatic hypotension, cirrohsis of the liver, spinal anesthesia, hemorrhage or shock and hypertension, these proposed studies may help to evaluate the contribution of these reflexes to the irreversible complications of these pathological disodrders. Ultimately, this could lead to the implementation of new therapeutic strategies for the treatment of patients with disorders which alter normal cardiovascular function.